1. Field of the Invention
This invention relates, in general, to power line communication systems and, more specifically, to carrier signal systems for distribution power lines.
2. Description of the Prior Art
Carrier communication systems for power lines have been in use for many years to transfer information between various points in the power system. However, most of the systems in use presently are confined to the transmission lines of the power system and do not span the secondary or distribution lines of the power system which lead to the customer location. There is considerable research and development presently being conducted in the area of communications between the customer location and locations along the transmission line facilities of the electric utility company. Both one-way and two-way communication systems have been described in the literature and are being operated to a limited extent.
In general, a two-way communication system has the capability of relaying information relative to the electrical power consumed by the customer, information concerning other quantities relative to the consumption of electrical power such as the rate of the consumption, the time in which the peak load is consumed, and the power factor at which the load is consumed, and other information which may be desired by the electric utility company in controlling and interpreting the electrical power used by the customer. Control signals may also be generated and transmitted from a remote location to the customer location to control the total amount of power used by the customer or to selectively disconnect certain electrical apparatus from the customer's electrical supply system under certain conditions, or for any other control reasons desired.
Attenuation along the power lines has been one of the major obstacles which exist in the way of standardized and reliable communication systems which span the entire power line network, including the secondary distribution lines which extend to the customer locations. It has been found that, for attenuation purposes, carrier frequencies in the range between 70 kHz and 90 kHz ensure good performance along the primary feeder lines of the power system. It has also been found that carrier frequencies in the range between 300 kHz and 350 kHz provide good performance along the secondary distribution lines of the power system. Consequently, to provide the best overall performance along the entire power system, it is necessary to change the carrier frequency at the junctions between the primary feeder lines and the secondary distribution lines. Normally, these junctions are usually at the location of a distribution transformer which, from experimental testing, has been found to be a source of high attenuation for the transmission of carrier communications.
In order to reduce the effects of the attenuation of distribution transformers, some communication systems use repeater or translator transmitters which are located at each distribution transformer location. These translators receive carrier communications on one frequency from the secondary distribution line and convert the intelligence to another carrier frequency which is applied to the primary feeder line. These translators also translate the carrier frequency received from the primary feeder line and apply the translated carrier to the secondary distribution line. The carrier frequencies used are those which exhibit the lowest attenuation characteristics for the particular power line to which the translators are connected.
Such translators include more than one transmitter to relay the communications around the distribution transformer. A typical translating system includes a high-power transmitter and a low-power transmitter which are both connected to the same location on a power distribution line. The high-power transmitter is characterized by a low impedance output. This low impedance output significantly affects the coupling of the low-power transmitter to the same distribution line even though the high-power transmitter is not transmitting when the low-power transmitter is turned on and transmitting. Thus, some type of switching or connecting arrangement is required to isolate the high-power transmitter from the low-power transmitter to permit effective use of the communication signal available.
Conventional relay devices would provide sufficient isolation between the two transmitters under normal operating conditions. However, the maintenance associated with such electromechanical devices and the limited number of switching operations are considered a deterring factor in the use of electromechanical relays in such power line communication systems. Thus, it is desirable, and it is an object of this invention, to provide a reliable and efficient means for coupling two carrier communication transmitters to the same location on a power distribution line.